The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Generally, exhaust gas flowing out from an engine through an exhaust manifold is driven into a catalytic converter mounted at an exhaust pipe and is purified therein. After that, the noise of the exhaust gas is decreased while passing through a muffler and then the exhaust gas is emitted into the air through a tail pipe. The catalytic converter purifies pollutants contained in the exhaust gas. In addition, a particulate filter for trapping particulate matter (PM) contained in the exhaust gas is mounted in the exhaust pipe.
A denitrification catalyst (DeNOx catalyst) is one type of such a catalytic converter and purifies nitrogen oxide (NOx) contained in the exhaust gas. If reducing agents such as urea, ammonia, carbon monoxide, and hydrocarbon (HC) are supplied to the exhaust gas, the NOx contained in the exhaust gas is reduced in the DeNOx catalyst through oxidation-reduction reaction with the reducing agents.
Recently, a lean NOx trap (LNT) catalyst is used as such a DeNOx catalyst. The LNT catalyst adsorbs the NOx contained in the exhaust gas when air/fuel ratio is lean, and releases the adsorbed NOx and reduces the released nitrogen oxide and the nitrogen oxide contained in the exhaust gas when the air/fuel ratio is rich atmosphere (hereinafter, it will be called a ‘regeneration of the LNT’).
Since general diesel engines are operated at the lean air/fuel ratio, however, it is required to artificially adjust air/fuel ratio to be the rich air/fuel ratio in order to release the adsorbed NOx from the LNT. For this purpose, a timing for releasing the NOx adsorbed in the LNT (i.e., regeneration timing). Particularly, the timing where the NOx adsorbed in the LNT can be released should be precisely determined to improve purification efficiency of the NOx and fuel economy and to inhibit degradation of the LNT.
The above information disclosed in this Background section is only for enhancement of understanding of the present disclosure and therefore it may contain information that is not already known to a person of ordinary skill in the art.